Make async execution the default

[kaqqao]

Since Spring MVC supports async responses, and graphql-java itself works asynchronously, we can safely go fully async and bring the default MVC executor in line with the WebFlux one.
The only downside is that it breaks existing tests using MockMvc. The options are to use asyncDispatch or use blocking execution in tests.

To restore the old blocking behavior, it is enough to add graphql.spqr.http.mvc.executor=blocking to application.properties. This is what we do in our own tests.

The affect of this is the same is as if new DefaultGraphQLExecutor(...).blocking() was registered as a bean, e.g.:

@Bean
public GraphQLMvcExecutor blockingExecutor(
                        MvcContextFactory contextFactory,
                        Optional<DataLoaderRegistryFactory> loaderFactory) {
    new DefaultGraphQLExecutor(contextFactory, dataLoaderRegistryFactory.orElse(null)).blocking();
}